Power transmission networks typically include interconnected electrical networks operating at different voltages. The variation in these operating voltages arises as a result of many factors including the size and locality of the individual electrical network, the local power requirements and so on.
In high voltage direct current (HVDC) power transmission networks, alternating current power is generated by the generator plant at a low voltage level in the range of a few kV before being stepped up in a collector station to a higher voltage level in the range of a few hundred kV and then converted to direct current power for transmission via overhead lines and/or undersea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the transmission line or cable, and thereby reduces the cost per kilometer of the lines and/or cables. Conversion from AC to DC thus becomes cost-effective when power needs to be transmitted over a long distance.
DC transmission and distribution networks are needed to support the emergence of HVDC power transmission. However, interconnecting the DC transmission and distribution networks to form a DC power grid is difficult because different DC networks may operate at different voltage levels depending on various factors as outlined above.
In order to interconnect power networks operating at different voltage levels, it is necessary to employ devices that are capable of performing a voltage step-up/step-down operation.
An example of such a device is a transformer. Transformers conventionally used in distribution, industrial and power applications typically include primary and secondary windings which are linked to separate power networks. Electrical power is transferred from the primary winding to the secondary winding by varying the current in the primary winding, and thereby creating a varying magnetic flux in the transformer. This change in magnetic flux leads to an induction of voltage in the secondary winding and thereby a transfer of electrical power between the separate power networks. As such, transformers are suitable for interconnecting separate AC networks. Transformers can also be employed in DC to DC voltage conversion, which involves conversion of a DC voltage to an AC voltage to enable the use of a transformer and converting the stepped-up/stepped-down AC voltage back to DC voltage.
Transformers used in high voltage applications however tend to be bulky and heavy. This not only adds to the overall size and weight of the power converter and power station, but also leads to increased costs associated with transport of the transformers to the site of the power station.
Other examples of devices that are capable of performing a voltage step-up/step-down operation are buck converters and boost converters. These DC to DC converters are forms of switched mode power supplies that rely on the use of switches and passive elements to control the transfer of power between DC networks. The voltage step-up/step-down capability of these DC to DC converters can be regulated to a desired level by controlling the duty cycle of the switches. In general, the structure of each of the buck and boost converters leads to inefficient transfer of power between the DC networks.
In addition, the flow of current in the buck and boost converters is such that their switches are required to be rated so as to be compatible with both high voltage, low current and low voltage, high current DC networks.
Similarly, AC to DC and DC to DC voltage conversion schemes, which employ the use of transformers, also require switches which are capable of supporting the entire power load during voltage conversion.
Consequently the switches must be designed to have both high voltage and high current ratings, which leads to an increase in hardware size, weight and cost.
Overview
According to a first aspect of the invention, there is provided a converter for use in high voltage direct and alternating current power transmission, the converter comprising a primary charge transfer converter, including first and second primary terminals for connection to one or more electrical networks, a plurality of charge transfer elements and a plurality of primary switching elements connected in a cascade circuit between the first and second primary terminals, each charge transfer element including at least one resonant circuit, the primary switching elements being operable to selectively cause charging and discharging of each resonant circuit to transfer charge between the charge transfer elements and thereby create a voltage difference between the first and second primary terminals
The provision of resonant circuits in the charge transfer converter allows power to be transferred between the first and second primary terminals of the charge transfer. Such transfer takes place by commutation of the primary switching elements which can occur at near zero current so as to minimise switching losses. The converter of the invention is therefore very efficient.
Additionally, during operation of the converter, a significant portion of the overall current flowing in the converter flows directly between the charge transfer elements rather than via the primary switching elements. Consequently each primary switching element only carries a small portion of the overall current flowing within the converter, which means that it is possible to use primary switching elements with lower power ratings. This leads to a decrease in hardware costs and in the physical size of the converter.
The use of a cascade circuit in the converter permits the interconnection of electrical networks having different voltage levels and thereby removes the need for large and bulky transformers to step up or step down the operating voltage. This in turn leads to a reduction in converter size, weight and cost, which is beneficial for locations having restrictions on converter size and weight such as, for example, offshore power stations.
The modular nature of the cascade circuit means that it is relatively straightforward to increase or decrease the number of charge transfer elements and primary switching elements. As such, the converter to the invention can be easily modified to suit the requirements of the associated power application, such as station footprint size or required voltage operating range.
Preferably, the junction between adjacent primary switching elements defines a secondary terminal.
The provision of one or more secondary terminals permits the converter to be simultaneously connected to multiple power networks having different voltage levels and also provides flexibility of being connectable to a wide range of voltages without having to modify the design and structure of the converter.
Optionally, the or at least one resonant circuit of each charge transfer element includes at least one inductor connected in series with at least one capacitor.
At least one charge transfer element may include a plurality of parallel-connected resonant circuits.
Such features allow the structure of each charge transfer element to be varied depending on the power ratings of available components and the voltage and current requirements of the associated power application.
Each primary switching element preferably is or includes a semiconductor device.
Each primary switching element may also include an anti-parallel diode connected in parallel with the semiconductor device.
The selection such of primary switching elements allows the converter to be configured to transfer power from the first primary terminal to the second primary terminal or vice versa.
The or each semiconductor device may be an insulated gate bipolar transistor, a gate turn-off thyristor, a field effect transistor, a transistor, an injection enhancement gate transistor, an insulated gate commutated thyristor or an integrated gate commutated thyristor.
The use of semiconductor devices is advantageous because such devices are small in size and weight and have relatively low power dissipation, which minimises the need for cooling equipment. Their inclusion, therefore, leads to significant reductions in power converter cost, size and weight.
Conveniently the plurality of primary switching elements define a cascade arrangement of alternating odd and even primary switching elements, the primary switching elements being controllable to selectively put each of the odd primary switching elements in a first open/closed state and each of the even primary switching elements in a second open/closed state opposite the first open/closed state.
The plurality of primary switching elements are controllable to alternate between a closed state and an open state.
The operation of the primary switching elements in the foregoing manner ensures that the majority of the charging and discharging current flows directly between charge transfer elements instead flowing through each primary switching element. This reduces the current load through each primary switching element during normal operation of the converter, and so reduces the corresponding current rating required for the switching element.
Preferably each of the primary switching elements is controllable in use to delay switching between the open state and the closed state.
Each of the primary switching elements is also preferably controllable to vary the length of the delay in switching between open and closed sates.
The delay in switching helps to ensure that there is no direct connection between the voltages connected to the first and second primary terminals at any time and thereby prevents short-circuiting of the charge transfer elements. There is, therefore, a reduced risk of a higher than normal current load passing through each primary switching element.
Preferably the resonant circuit of each charge transfer element is tuned to the same resonant frequency.
Optionally the switching frequency of each primary switching element is approximately equal to the resonant frequency of the resonant circuits.
This leads to the formation of a sinusoidal current in the primary charge transfer converter, which enables soft switching of each primary at or near zero current, switching element and thereby reduces switching losses in the primary switching elements.
The converter may further include least one DC link capacitor connected in parallel with the primary charge transfer converter.
The inclusion of a DC link capacitor improves the efficiency of the voltage conversion process by minimising harmonic distortion in a DC voltage from the first DC network.
In a preferred embodiment of the invention the converter further includes a primary auxiliary unit connected to the second primary terminal thereof, the primary auxiliary unit defining either a charge store or a charge generator including an auxiliary terminal for connection to an electrical network.
The provision of a primary auxiliary unit allows the converter to provide a voltage step-down or step-up without the need for a bulky and expensive transformer.
Conveniently the primary auxiliary unit defines a charge store including at least one reservoir capacitor to store said charge.
Such a feature allows the converter to provide a voltage step-down.
Optionally the primary auxiliary unit defines a charge generator including an oscillator circuit to introduce an AC voltage component into the charge transfer elements of the primary charge transfer converter. This allows the converter to provide a voltage step-up.
In another preferred embodiment of the invention the oscillator circuit operates at a frequency that approximates the resonant frequency of the charge transfer elements.
Such an arrangement allows switching of the primary switching elements at or near zero voltage, and so minimises losses in the converter.
According to a second aspect of the invention there is provided a converter assembly comprising a converter as described hereinabove, the first primary terminal of the primary charge transfer converter being connected in use to a positive terminal of a first DC network, the auxiliary terminal of the auxiliary unit being in use to a negative terminal of the first DC network, and the junction between respective adjacent primary switching elements being connected in use to respective positive, negative and ground terminals of a second DC network.
According to a third aspect of the invention there is provided a converter assembly comprising a first converter as described hereinabove, a plurality of parallel-connected secondary charge transfer converters each having a respective secondary auxiliary unit connected to the second primary terminal, the first primary terminal of each parallel-connected secondary charge transfer converter being connected to the auxiliary terminal of the first converter.
The converter assembly may further include a transformer connected to the auxiliary terminal of each secondary auxiliary units and a respective phase of a multiphase AC network.
Optionally the converter assembly includes a second converter as described (hereinabove), the auxiliary terminal of the second converter being connected to the auxiliary terminal of each secondary auxiliary unit the first primary terminal of each converter being connected in use to respective positive and negative terminals of a first DC network, and each secondary charge transfer converter including a secondary terminal for connection in use to a respective phase of a multiphase AC network.
According to a fourth aspect of the invention there is provided a converter assembly comprising a plurality of converters as described hereinabove, the converter assembly including at least one converter limb having first and second limb portions, each limb portion including a said converter, the auxiliary terminal of the auxiliary unit in the converter in the first limb portion being connected in series with the auxiliary terminal of the auxiliary unit in the converter in the second limb portion to connect the converters in a given convert limb with one another end to end the first primary terminal of the converter in the first limb portion being connected in use to a positive terminal of a first DC network, the first primary terminal of the converter in the second limb portion being connected in use to a negative terminal of the first DC network, and the series connection between the auxiliary terminals defining a phase terminal connected in use to an AC network, the auxiliary unit of each limb portion being operable to switch the respective limb portion in and out of circuit so as to generate a voltage waveform at the phase terminal of the respective converter limb.
Such a converter assembly preferably includes a plurality of converter limbs, each converter limb defining a phase terminal for connection to a respective phase of a multiphase AC network.
Each converter limb operates independently of the other converter limbs and therefore only directly affects the phase connected to the respective phase terminal. As a result a given converter limb causes minimal disruption to the phases connected to the phase terminals of the other converter limbs.
As set out above the converter of the invention can be easily incorporated into various converter assemblies to specific requirements of the associated power application.
Preferably each auxiliary unit includes reference terminal which is connected in use to a lower voltage potential than the corresponding auxiliary terminal.